1. Field of the Invention
Embodiments of this invention relate generally to the measurement of dynamic coercivity effects in magnetic media, and in particular embodiments to methods for non-destructive measurement of dynamic coercivity effects in magnetic media, and systems incorporating the same.
2. Description of Related Art
Modern computers require media in which digital data can be quickly stored and retrieved. Magnetizable (hard) layers on disks have proven to be a reliable media for fast and accurate data storage and retrieval. Disk drives that read data from and write data to hard disks have thus become popular components of computer systems.
As the recording media industry strives for increasingly smaller disk drives with increasingly larger storage capability, the areal density (the number of bits, or magnetic flux reversals, per inch) of magnetic disks continues to increase. In order to maintain a sufficient signal-to-noise ratio, the number of magnetizable units in each stored bit must be kept above a certain minimum value. To reduce bit size while maintaining a certain number of magnetizable units in each bit, the size of the magnetizable units (the particle or crystal grain size) must therefore be correspondingly decreased.
However, very small particles may be magnetically unstable due to the effect of thermal agitation (onset of superparamagnetism). Typically, an energy barrier inherent in a magnetized particle prevents the particle from reversing its magnetization, but as particle size decreases, this energy barrier also decreases. As the energy barrier becomes increasingly small, the likelihood that a particle will spontaneously reverse its magnetization due to thermal agitation over a given time period increases. This state, known as superparamagnetism, is characterized by low coercivity (the ability of a particle to resist magnetic change) and low remanent coercivity (the magnetic field required to reduce the magnetization of the particle to zero), and results in a particle with high magnetic instability.
Particle coercivity is also dynamic. Generally, particle coercivity is high when measured over short periods of time, but is low when measured over extended periods. Low coercivity over extended periods of time may cause the magnetic media to demagnetize. In addition, as the superparamagnetic regime is approached, the timedependence of coercivity increases. Thus, the related factors of small particle size, low coercivity, and the onset of superparamagnetism cause recorded bits to become unstable, increasing the likelihood that stored information will be lost.
The time-dependence of coercivity is particularly relevant to magnetic recording media because of the need to support both short-term writing capability and long-term data storage stability. An apparent solution to the problem of magnetic instability would be to utilize a magnetic material with a larger anisotropy constant to increase the "storage" or "long-term" coercivity until the desired stability of the magnetically stored information is achieved. However, this may also cause a significant increase in short-term coercivity and consequent writing difficulties. A balance between short-term and long-term particle coercivity must be achieved to produce both reliable data writing and stable data storage. The measurement of dynamic (time-dependent) coercivity effects in magnetic media is therefore an important capability in the design and development of magnetic recording media.
Conventional methods for measuring dynamic coercivity effects have employed destructive techniques for making the measurement. In these methods, test coupons or samples are separated from the magnetic media and placed in a test fixture for analysis by a magnetometer. For example, the article "Reptation effects in particulate systems. II. Experimental studies" by Lewis, et al., published in the Journal of Applied Physics, Vol. 73, No. 10, May 15, 1993, discusses the measurement of the time-dependence of coercivity using a sample of magnetic film and an alternating gradient force magnetometer (AGFM). In this approach, a sample must first be taken from the magnetic media and placed in a separate tester. The necessity of taking a sample physically destroys the magnetic media so that no further measurements can be made.